In conventional internal combustion engine arrangements in use today, fixed drive mechanical water pumps are currently commonly used to pump coolant through the engine so as to prevent overheating of the engine. In such arrangements, drive power from the engine is transferred from the engine crankshaft via a crankshaft pulley to several accessories (including a water pump) via an endless drive member, such as a Poly-V or multi-ribbed rubber drive belt. Typically, the speed of the belt-driven water pump (i.e. the speed of the water pump's impeller) is directly related to the speed of the engine due to their connection via the accessory drive belt.
It is known that engine cooling requirements vary depending on many factors, including the engine load and the speed of the vehicle. Typically, a fixed drive water pump as described above must be designed to cool the engine sufficiently under the worst conditions such as a situation in which the engine is at low RPM, at high engine load, and at low vehicle speed). However, due to the fixed connection between the water pump and the engine, this means that in many situations the amount of cooling supplied by the pump is more than is necessary.
Such a situation robs the engine unnecessarily of power, which means that the engine must make more power than would otherwise be necessary in order to drive the vehicle. Furthermore, if the engine is kept too cold, incomplete or imperfect combustion takes place, resulting higher than necessary emissions from the vehicle. Thus, by overcooling the engine in some situations, the emissions of the vehicle can be higher than desired, in addition to whatever increase in emissions results from the increased amount of power the engine must develop in order to drive the water pump even when the water pump is not needed.
Electrically powered, variable speed water pumps have been used in some instances, however, such pumps are typically expensive and they impose an additional electrical load on the vehicle's battery, which is not always desirable.
A wrap spring clutch that is controlled by an actuator has been proposed, which engages and disengages the belt drive from the water pump as needed, so as to reduce the overall power consumption of the water pump. While this is successful, other ways of controlling the engagement of the water pump would be beneficial.